A joined body includes: a thin plate including through holes; first members each including: a base portion and a protruding portion inserted into one of the through holes; and a second member placed oppositely to the base portion via the thin plate and made of a material that is of a same kind as a material of the first members. The thin plate is made of a material having a specific gravity smaller than the materials of the first and the second members. The second member and an end portion of the protruding portion are connected together by melt-solidification. A post-joining hardness difference in a range of 30% along a direction orthogonal to a joint interface between the second member and the protruding portion, the range being centered on the joint interface, is equal to or less than 90% of a pre-joining hardness difference.